Array antennas are arrangements of antenna elements working in together in concert to provide higher power handling, higher gain, higher directivity with lower sidelobes than is generally possible with singular antenna elements or even non-array arrangements of antenna elements. Additionally, they permit dynamic directional steerability under electronic control which is also an attribute not generally found in singular antenna element instantiations. Array antennas have numerous vital applications in radar imaging, target tracking, sensor data collection, and precision location and have more recently found application in numerous new high technology applications such as medical imagining, RF and optical astronomy, and Ultrasound.
Although array antennas have numerous wonderful attributes for numerous applications, they almost universally suffer from three common limitations or maladies. First, is the limitation on low frequency operation due to element cutoff, second is the limitation on high frequency operation due to grating lobe formation, and third is the resultant small limited bandwidth resulting from these two other limits. It is a key goal of the present invention to solve all three of these most challenging problems all at once.
The first limitation on low frequency cutoff is due to the finite size of the antenna elements making up the antenna array. The elements making up an array are still limited by the laws of antennas physics to a low frequency cutoff equating to when the physical size of the element is about a third of the low frequency cutoff wavelength (lambda/3). This is a fairly hard law to break and is quantified by the McClean-Chu-Harrington limit and their several variations. To the extent that antenna elements might be made smaller, they will invariably be of tower radiation efficiency which is antagonistic to most array performance specifications.
The second limitation concerns the generation of grating lobes if the wavelength used becomes shorter than twice the inter-element spacing (lambda/2). Grating lobes are almost universally bad because they channel power in directions other than the intended direction. This both puts signal power where it might do harm (alerting an enemy to a radar's presence for example) and at the same time robs power from the desired direction by diverting it to other unintentional directions
The confluence of these two limitations above result in a lower frequency limit defined by the element cutoff, and high frequency limit defined by the formation of grating lobes, the usable frequencies in between define the usable bandwidth. Given that the low frequency cutoff of the elements is about a third of a wavelength and the high frequency formation of grating lobes occurs at about half a wavelength, this limits the bandwidth of an array antenna to something less than about 40% bandwidth, with 30% being a more typical number due to the restrictions imposed by other related limitations.
Although these may sound like reasonable bandwidth fractions based on past antenna and array requirements, new technology advancements are requiring octave and even decade bandwidth from antennas, and they are also required to retain all the other typical performance metrics such as being highly efficient, high power handling, low cost, producible, etc. The current array antenna simply does not support these new requirements and therefore a new advancement is needed in the area of wideband high power electronically steerable array technology. It is therefore the goal of this invention to address this need with a new array technology that can actually meet all these stressing new requirements simultaneously while also being low cost, rugged and producible.
With these applications, such an antenna would be superior to alternate antennas and antenna configurations for a variety of reasons including its ability to share bandwidth spectrum with other users, its immunity to multi-path fading, and its manifestation of both clear and improved signal reflection.
Still, while antenna array advantages generally outweigh disadvantages, there are downsides. For instance, previous arrays have required resistive loading (e.g., R cards) to insulate against radiation resulting from back reflection which would otherwise degrade the Return Loss (VSWR). The present invention minimizes back reflection, thus minimizing the need for lossy loading treatments.
Further, traditional array antennas, because of an after the fact difference between their feed and antenna impedance, require impedance transformers, which can add significant loss to the system, limit bandwidth, limit power handling, introduce phase and frequency distortion, increase the space consumed by the antenna, increase cost and reduce reliability. By controlling impedance organically within the antenna proper through the explicit design of the antenna architecture, contours, shaping and structure, the present invention creates less of a mismatch between the feed and antenna impedances, resulting in both a better voltage standing wave ratio (VSWR) match and superior space management. In addition, the ability to actually design the impedance of the antenna to be what ever value the designer might choose, allows one to design the antenna feed impedance to be a common value (e.g. 50, 75 or 100 ohms) enabling the use of low cost readily available commercial of the shelf (COTS) components, eliminating the traditional array need for expensive custom components, thus decreasing cost and time to market.